The present invention relates to the field of radio frequency probe or coil design for magnetic resonance imaging and spectroscopy. The present invention finds particular application in conjunction with probes or coil designs which are not circularly symmetric and will be described with particular reference thereto. However, it is to be appreciated that the present invention is also applicable to circularly symmetric probes or coils in which conductor placement is not fully symmetric relative to all axes.
NMR probes are most commonly constructed either of wide foil sheets or a small cross section of wire conductors. The foil conductor probes include slotted tube resonators and Adelman-Grant resonators. In foil probes, the current distribution on the foil is determined by electromagnetic laws governing the nature of sheet currents and are not necessarily uniform. The wire type probes are exemplified by saddle coils and loop-type surface coils. Unlike foil probes which provide the designer with relatively little design discretion to adjust current densities, wire probes are readily adjustable. In wire type probes, the designer can shape the current distribution by choosing the proper location for conductors. For example, in saddle coils, angular locations of 120.degree. and 60.degree. have been determined to yield the best transverse uniformity. Hybrid probes in which wire type coils are constructed of foil strips in a saddle, loop, or like configuration, provide a compromise between the advantages of foil type probes and the design flexibility of wire type probes.
Previously, many efforts have been made to optimize the field uniformity of NMR probes. The geometries of single turn saddle coils and slotted tube resonators along a circular cylinder have been optimized for various diameters. Multi turn saddle coils provide increased uniformity for low frequency magnetic resonance imaging, but stray capacitive effects limit their utility at higher frequencies.
Among the most uniform probes is the bird cage coil illustrated in U.S. Pat. Nos. 4,680,548 and 4,694,255. In bird cage coils, a plurality of conductors are mounted longitudinally at equal spacings on the surface of a circular cylinder. Each conductor is interrupted by a capacitor, each of which has the same capacitance. Field uniformity is improved by increasing the number of symmetrically disposed conductors. The amplitude of the parallel conductor currents are weighted as a cosine function which yields an optimum uniformity. This optimum weighting is the result of the symmetrical arrangement of identical longitudinal conductor branches. Another drawback to the bird cage coil resides in the complexity of fine tuning. In order to maintain the symmetry around the circular cylinder, during tuning it is necessary for all capacitors to be adjusted to the same capacitance value.
Another approach for optimizing field uniformity is illustrated in U.S. Pat. No. 4,591,818 to Peter C. Butson. A plurality of conductors are arranged at spaced positions around a circular cylinder. The conductors, whose positions are mirror imaged about two orthogonal axes, have different cross sections. The conductors are arranged at precise 30.degree. intervals and have different diameters. This difference in conductor diameter changes the relative impedance of the conductors, hence the amount of current flowing therethrough.
The present invention provides a new and improved method of designing coils with uniform fields which allows the coils to be non-circular or otherwise unsymmetric and the coils so designed.